The present invention relates to mercury vapor discharge lamps and more particularly to mercury vapor discharge lamps including a reflector layer.
Various coatings of non-luminescent particulate materials have been found to be useful when applied as an undercoating for the phosphor layer in both fluorescent and other mercury vapor lamps. In both types of lamp, the phosphor coating is disposed on the inner surface of the lamp glass envelope in receptive proximity to the ultraviolet radiation being generated by the mercury discharge.
Examples of non-luminescent particulate materials which have been used as reflector layers in fluorescent lamps such as, for example, aperture fluorescent reprographic lamps, include titanium dioxide, mixtures of titanium dioxide and up to 15 weight percent aluminum oxide; zirconium oxide; aluminum oxide; aluminum; and silver. Titanium dioxide is typically used to form the reflector layer in commercially available aperture fluorescent reprographic lamps.
In some instances a layer of non-luminescent particulate material is used to permit reduction in the phosphor coating weight. See, for example, U.S. Pat. No. 4,079,288 to Maloney et al., issued on 14 March 1978. U.S. Pat. No. 4,074,288 discloses employing a reflector layer comprising vapor-formed spherical alumina particles having an individual particle size range from about 400 to 5000 Angstroms in diameter in fluorescent lamps to enable reduction in phosphor coating weight with minor lumen loss. The lamp data set forth in the patent, however, shows an appreciable drop in lumen output at 100 hours.
U.S. Pat. No. 4,344,016 to Hoffman et al., issued on 10 August 1982 discloses a low pressure mercury vapor discharge lamp having an SiO.sub.2 coating having a thickness of 0.05 to 0.7 mg/cm.sup.2. U.S. Pat. No. 4,344,016 expressly provides that the use of thicker coatings causes a reduction in the luminous efficacy due to the occurrence of an absorption of the visible light.